1. Field of the Invention
The present invention relates to a light projection optical system and a light projection apparatus including the system.
2. Description of the Related Art
A distance measurement apparatus is known in which a luminous flux is projected to a predetermined region (a target region) with a light projection apparatus to measure a distance to a certain object in the target region from information of reflected light of the luminous flux. The light projection apparatus of a scanning type is known in which the projected luminous flux is scanned to move the projection region so as to acquire distance information of the large target region.
A light projection optical system mounted on such a light projection apparatus is demanded to efficiently condense light emitted by a light source and obtain an appropriate intensity distribution of the projected luminous flux. For example, if the intensity distribution excessively enlarges, information of an object other than a target to be measured is included in the reflected light as a noise component. This causes a drop in measurement precision. On the other hand, if the intensity distribution is excessively small, only information of a specific portion of the target can be obtained, and this results in lack of information.
Moreover, in addition to the function of scanning the projection region, there is sometimes a demand for a function of moving the projection region to an arbitrary position to fix the region at the position. Examples in which such a function is utilized are considered to include a case where a movement situation of a specific target is measured and an application in which detailed information of the target is acquired.
On the other hand, it is demanded that the light projection optical system and the light projection apparatus on which this system is mounted be miniaturized and lightened. It is also demanded that energy be saved and that luminance be increased.
Moreover, in recent years, as the light sources of the light projection optical system, a semiconductor laser and a light emitting diode (LED) have been used. These light sources have high output and long life.
In general, when a numerical aperture (NA) on a light source side is increased, more light emitted from the light source can be taken. To increase the NA on the light source side, a projection lens needs to be arranged close to the light source, or a diameter of the projection lens needs to be increased. However, when interference with a member or the like constituting a light source section is considered, there is a restriction in arranging the projection lens close to the light source. On the other hand, when the diameter of the projection lens is increased, a scanning member and the like have to be increased in size. This obstructs the miniaturization.
In Japanese Patent Application Laid-Open No. 8-297255, a scanning type light projection optical system is described in which the light source is once imaged and the NA on the light source side is increased to secure a quantity of light.